1. Technical Field of the Invention
The present invention relates to AM/FM converters and optical signal transmission systems used for optical transmissions of wideband signals. More specifically, the present invention relates to AM/FM converters and optical signal transmission systems which are suitable for optical transmissions of frequency-division-multiplexed amplitude-modulated multi-channel video signals, and are especially suited to use in optical subscriber systems based on passive-double-star (PDS) topologies.
2. Background Art
At present, many countries are vigorously pursuing research and development of optical transmission formats for transmitting wideband electrical signals by conversion into optical signals, as methods for transmitting multi-channel video signals. An optical transmission format wherein a semiconductor laser is intensity-modulated in proportion to the amplitudes of amplitude-modulated multi-channel video signals (hereinafter referred to as the AM-TV transmission format) is principally used for mainline optical transmissions of cable television. However, since the AM-TV transmission format has a low resistance to noise, the transmitting/receiving level difference cannot be too large; thus, the transmission length and the number of optical splitters in the optical transmission system are restricted.
In order to resolve this problem, an optical transmission format has been developed wherein a semiconductor laser is intensity-modulated based on video signals which are frequency-modulated separately by channel (hereinafter referred to as the FM-TV transmission format). Since the FM-TV transmission format with separate video channels is highly resistant to noise, the transmission length and the optical splitter number can be made large in an optical transmission system with a large transmitting/receiving level difference. However, this format has the drawback that the channels must be selected for demodulation, so that the receiver becomes expensive due to the need for complicated wideband channel selection circuits.
On the other hand, a format has been developed wherein a semiconductor laser is intensity-modulated based on multi-channel video signals which are frequency-modulated by batches over multiple channels and demodulated by batches into multi-channel video signals (hereinafter referred to as the batched FM-TV transmission format). The batched FM-TV transmission format is highly resistant to noise, so that the transmission length and the optical splitter number may be large in optical transmission systems with large transmitting/receiving level differences. Moreover, the optically transmitted signals are wideband but may be demodulated by simple circuits using high-speed ICs. Thus, the channels can be selected by means of normal cable-TV tuners, so that the receivers can be made cheaply. However, modulators which frequency-modulate multi-channel video signals by batches conventionally use voltage controlled oscillators (hereinafter referred to as VCOs), so that the frequency band for modulation is limited by the restrictions to the input frequency band of the VCOs. The highest frequency able to be handled by vCos is 200 MHz, because the input impedance to the VCOs increase with increased frequencies. Assuming that the transmission bandwidth for each channel of a television broadcast is 6 MHz and that the frequency bandwidth usable for transmission of television broadcasts is 90.about.210 MHz, then the number of video channels which can be simultaneously transmitted when using VCOs is about 20 channels. Furthermore, it is difficult to manufacture VCOs which maintain linearity over wide bands.